If you test "positive" for pot, they will take your kids.
I would hire an attorney immediately.
You need to find someone experienced in "fighting" CPS. Quite frankly, there are not many attorneys out there that have experience in CPS cases since most people can't afford to pay to fight CPS.
The previous poster is correct. I do fight CPS on a regular basis. The answer to your question depends on whether the case is in court or not. Many times, if the case is in investigation or in family based services (FBS) they may ASK you to submit to a drug test, and you have the right to refuse. Of course, that often leads to the case being filed in court, and, perhaps, to removal of the children. It is a very serious matter, and you should seek legal counsel to further guide you through this maze.